Interconnection cables between electronic apparatus that carry data and control signals at high frequency clock rates may generate electromagnetic radio frequency interference by acting as efficient antennas that effectively radiate undesirable radio frequency signals, e.g., radio frequency interference that may interfere with other electronic equipment that are designed to receive radio frequency signals, e.g., televisions, radio receivers, cordless telephones, etc. Electronic apparatus printed circuit boards without a separate ground plane layer may also generate undesired electromagnetic radio frequency interference. The Federal Communications Commission (FCC) has strict rules regarding the levels of electromagnetic radio frequency interference that may be emitted from electronic apparatuses.
Electromagnetic radio frequency interference may be effectively reduced by shielding interconnection cables, and for printed circuit boards having a continuous ground plane layer in close proximity to high frequency signal lines thereon. Shielded interconnection cables can be more expensive and are difficult to implement when using ribbon or flex cables. Ribbon and flex cables may comprise a plurality of conductors insulated from one another and arranged in a side-by-side planar configuration. According to an electromagnetic interference reduction technique described in “Introduction to Electromagnetic Compatibility” by Clayton R. Paul, hereby incorporated by reference herein for all purposes, the optimal number of grounded conductors for electromagnetic interference reduction is equal to the number of signal conductors plus one. This configuration allows for a grounded conductor—signal conductor—grounded conductor arrangement and is also effective in reducing radiated electromagnetic radio frequency interference from a printed circuit board not having a continuous ground plane layer. This arrangement may allow for close electromagnetic coupling of an unshielded signal line to associated ground return lines and may effectively reduce radiated electromagnetic radio frequency interference. However, the cost and space necessary for the increased number of conductors can be prohibitive and a lesser number of ground return lines are generally used. Use of less than the optimal number of ground return lines may significantly increase the amount of electromagnetic interference radiated by the cable and/or printed circuit board.